If you are doing these actions outside of the crop tool, then the app will choose what portion of the image to use for the 3840×2160, and it is probably the left part of the image.
I suggest this instead.
1. Open the crop tool
2. Set the aspect ratio to 16:9
3. Type the size into the text fields
4. Position the crop box.
Or:
1. Open the crop tool
2. Set the aspect ratio.
3. Position the box
4. Export but choose a custom size of 3840 x 2160.